An automatic paper feeder mounted to, for example, a copying machine includes a pick roller which takes out papers one by one from a stack of papers placed on a paper feed tray, and a feed roller for conveying the papers that are taken out by the pick roller. The pick roller is arranged to come into contact with the leading end of the lower surface of the lowermost paper of the stack of papers placed on the paper feed tray, and is rotated in a predetermined direction to take out the papers.
To take out the papers one by one, a pick spring and a separator pad are arranged to come into contact with the pick roller from the upper direction. The separator pad has a width narrower than the pick roller and is arranged nearly at the center in the axial direction of the pick roller. The pick spring has the shape of a fork with its two legs arranged to come into contact with the pick roller on both sides of the separator pad. The pick spring pushes the paper onto the pick roller, so that the lowermost paper of the stack of papers is brought into frictional engagement with the pick roller; i.e., as the pick roller rotates, the paper is drawn by the pick roller due to frictional force. The separator pad is made of a rubber sheet or the like, comes into frictional engagement with the upper surface of a second paper from the lower side which is not in direct contact with the pick roller, and prevents the second paper from advancing together with the paper that is being taken out. Thus, the lowermost paper only is separated and is taken out while preventing a plurality of other papers from being taken out overlapped one upon the other.
In the automatic paper feeder, a variety of papers having different thicknesses and different stiffnesses are used. It is therefore desired that the automatic paper feeder is capable of separating and taking out a variety kinds of papers reliably one by one. Furthermore, in the automatic paper feeder, a failure in picking papers may occur when the amount of papers stacked on the paper feed tray decreases.
In order for the papers to be reliably taken out up to the last piece without picking miss, it is desired that a position (picking position) at which the pick spring comes into contact with the pick roller is located slightly on the upstream side of a position (separating position) at which the separator pad comes into contact with the pick roller in the direction in which the paper is fed. In this case, however, a gap is formed between the picking position and the separating position, and an increase in the gap invites an occurrence of paper jamming between the picking position and the separating position. Therefore, difficulty is involved in determining whether the pick spring and the separator pad be arranged at positions close to each other or whether they be arranged at positions remote from each other in the stage of designing. So far, the picking position has been located on the upstream side of the separating position by only a small amount in the direction in which the paper is fed, so that the papers are reliably taken out up to the last piece resulting, however, in the occurrence of pick miss as the amount of papers decreases. Moreover, the pick spring and the separator pad are interfered by each other in regard to their positions when they are arranged at positions very close to each other in the direction in which the paper is fed. To avoid this, therefore, the pick spring has been formed in the shape of a fork with its both legs being located on both sides of the separator pad.
In order to reliably separate the paper by the frictional force, furthermore, the separator pad must be brought into the pick roller with a strong force of contact, or the separator pad must be lengthened to increase the area of contact. When the force of contact becomes too great, however, the paper enters into between the pick roller and the separator with difficulty. When the contact area increases, the paper fails to come into uniform contact with the separator pad in the direction of width thereof due to error in the machining or backlash, causing the paper to be skewed.
In a document input device having an automatic paper feeder, furthermore, an optical read unit including an image scanner and the like is provided in the main device (frame) and is allowed to move along a carrier axis by a belt or the like. A glass plate is secured to a top plate of the main device to place a paper (document) thereon. With the paper (document) being placed still on the glass plate, the data are read out by an optical read unit. The automatic paper feeder may also work to convey the paper (document) to an end region of the glass plate to read the data by the optical read unit while the paper (document) is moving. The frame of such a device must have a sufficient degree of strength to support the moving optical read unit cutting off vibration. It is further desired that the top plate having glass plate is on a plane in parallel with the optical unit, that the top plate having glass plate and the carrier shaft are so arranged as to maintain a correct positional relationship, and that the device can be produced at a reduced cost.
Moreover, the frame of the device undergoes expansion and contraction depending upon the temperature. The frame that is substantially made of a metallic material only undergoes thermal expansion and contraction little. Though the cost can be decreased if the frame is partly made of a resin-molded article, it is desired that the top plate of the frame is made of a metallic material to maintain stiffness. When the frame is made up of a metallic material and a resin material that are arranged being facing each other, there develops a difference in the thermal deformation between the two to a degree that cannot be neglected, which turns out to be error in the reading of data.